When the bonnet structure of a conventional high pressure gate valve is assembled to the valve body, the bolts or studs and nuts that interconnect the body and bonnet structures are typically stressed quite highly in order to prevent leakage at the bonnet joint during periods when the valve is pressurized to its maximum operating pressure. Moreover, when the valve body is pressurized, these studs and nuts and bolts are subjected to considerable stress due to pressure acting on the seal area of the bonnet-to-body connection. In the event of a severe fire in close proximity to the valve and actuator mechanism, the studs and nuts or bolts that secure the bonnet to the valve body will become heated quite rapidly. In many cases, valve failure occurs during the excessive heat of direct flame impingement because the studs and nuts become heated to a temperature that reduces the strength thereof to the failure point. In many cases, it is desirable that valves be designed to remain safe and operative for extended periods of time even under circumstances where direct flame impingement causes rapid heating thereof. It is desirable, therefore, to prolong the time required for the studs and nuts of a bonnet-to-body connection to reach a temperature that reduces the strength of the studs and nuts to a level where failure occurs and the bonnet seal is lost. By prolonging the typical failure time of conventional valves, it is possible for the external fire to be extinguished before any high pressure valves can become heated to the point that stud and nut failure occurs. Obviously, in the case of combustible materials, such as petroleum products, it is highly desirable that fire induced valve failure be retarded in order to prevent the high pressure petroleum products from leaking from the bonnet connection and feeding the fire.
Another typical cause of valve failure induced by the heat of external fires is the failure of the stem packing structure of the valve which ordinarily prevents internal valve pressure from escaping from the valve stem opening of the bonnet. Under circumstances of excessive heat, typical stem packing materials deteriorate quite rapidly and tend to allow stem leakage. It is desirable, therefore, to provide a valve and valve actuator mechanism having the capability of retarding transfer of the heat of an external fire to the valve stem packing and thereby retard any valve stem leakage that might otherwise occur. It is also desirable to provide the packing structure of the valve with means to promote introduction of a combination lubricant and sealant material by way of an exposed lubrication fitting even though the valve and valve actuator mechanism may be designed for retarding transfer of heat to the bonnet structure of the valve.
In many cases, pneumatic valve actuators are employed incorporating spring return features that induce mechanical movement of a valve stem to a predetermined position in opposition to the direction of movement induced by a pneumatic diaphragm controlled system. One of the problems with spring return of pneumatic diaphragms, however, is the degree of rotary force to which the diaphragm is often subjected as the compression spring winds and unwinds during compression and extension thereof during linear movment of the valve stem. In many cases it is also desirable to prevent introduction of rotational forces to the valve stem as well. It is desirable, therefore, to provide a valve actuator mechanism incorporating means to protect the diaphragm of the pneumatic actuator from spring-induced rotational forces during actuation thereof.
In some types of diaphragm type actuators, an actuator stem is threaded to the valve stem so that upon rotation of the actuator stem (e.g., manually), the valve stem is moved axially to open or close the valve. In such instances, it is desirable to minimize the torque applied to the diaphragm by the rotating actuator stem.
One of the serious disadvantages of pneumatic actuators, as well as many other types of valve actuators, is the inability of the valve actuators to be repaired and/or replaced while the valve associated therewith remains under pressure. Under circumstances where it is simply necessary to replace the diaphragm of a diaphragm type pneumatic actuator, it is frequently necessary to shut down the entire production line and deplete the pressure of the valve controlled by the actuator before the actuator can be disassembled for replacement or repair. It is desirable, therefore, to provide a pneumatic valve actuator mechanism that may be simply and efficiently removed for replacement or repair without necessitating complete shutdown and depressurization of the flow system that is controlled by the valve.